This invention is concerned with the production of magnesium metal by the metallothermic reduction of magnesium oxide at elevated temperatures. More particularly, it relates to an improved process of the type in the prior art wherein metallic silicon, customarily in the form of a ferrosilicon alloy, and magnesium oxide, customarily in the form of calcined dolomite, are caused to react in an electric furnace-condenser system, customarily maintained under a high vacuum.
In such a process, known as the "Magnetherm" process, the oxidation of the silicon and the reduction of the magnesium oxide take place in the presence of a molten slage bath at temperatures above about 1300.degree.-1400.degree. C. In this process, the reaction zone is maintained under vacuum, since the magnesium product is removed from the reaction zone by distillation; see generally "Magnesium by the Magnetherm Process", C. Faure et al., J. Metals (Sept. 1964), pp. 721-23. Under the high vacuum, magnesium vapor at very low partial pressure evolves and is subsequently condensed to molten or solid magnesium metal. Periodically, "spent" ferrosilicon alloy and a large quantity of slag are tapped from the furnace in molten form. Since the furnace-condenser system is maintained under high vacuum in order to promote the desired reaction and to vaporize the magnesium product, the periodic removal of slag and spent alloy from the furnace requires that the vacuum be broken and the operation interrupted. The process is therefore essentially a batch operation.
The equipment in which such a process has been carried out comprises in sequence: feed bins in which the raw materials are stored and from which they are fed to the furnace through ducts or tubes; the furnace proper in which the reducing reaction takes place; a throat or duct through which magnesium vapor released in the reaction zone passes; a condenser in which the magnesium vapor is condensed to molten metal; and a pot or crucible attached to the condenser in which the molten magnesium is collected.
In such an operation it is necessary to remove the molten products from the system periodically. Tapping the furnace under high vacuum to remove molten slag and spent reductant is impractical, and it is difficult to remove molten magnesium from the crucible under high vacuum. The procedure used in the Magnetherm process is periodically to stop the reaction and then to break the vacuum, thus permitting the furnace to be tapped and the crucible removed. As soon as the furnace is plugged and the crucible replaced by a fresh one, vacuum is once more applied, and the operation is started again.
Such a process has been operated only on a small commercial scale, in contrast with the method now used for the production of magnesium on a large commerical scale, namely, the electrolysis of molten magnesium chloride.
Another metallothermic process for the production of magnesium, the "Pidgeon" process, is also a batch operation which has been operated only on a comparatively small commercial scale. In this process magnesium oxide, in the form of calcined dolomite, and metallic silicon, in the form of ferrosilicon, are charged into batteries of small externally heated retorts, in which a "solid state" reaction occurs at temperatures on the order of 1100.degree.-1200.degree. C. Under these conditions metallic magnesium is released from the reaction zone as a vapor at very low partial pressure, and it is therefore necessary to maintain the battery of retorts under very high vacuum.
In my earlier application, Ser. No. 796,214, there is disclosed a significant improvement to the Magnetherm process which permits operation at a relatively high pressure. An inert gas at a partial pressure of at least 0.1 atmosphere is provided above the molten slag in order in order that the absolute pressure of the system may be increased. In the development of this earlier invention, I have found not only that such operation at relatively high pressure is possible, but also that in certain instances unexpected benefits may be attained. These are the results of the instant invention, which can be characterized as an improvement upon my earlier invention.
One early reference, W. Moschel et al., "Magnesium" Chemische Technologie, vol. 5 pp. 102-64 (Winnacker & Weingartner, ed.) (1953), describes an attempted use of an inert gas, hydrogen, in the vapor space above a reaction zone where solid dolomite is reduced by silicon (p. 145). The conclusion then reached was that in such a system it was necessary to operate above the temperature at which the magnesium reaction pressure is one atmosphere, in order to carry the reaction to completion, otherwise the reaction would stop. The use of a stream of inert gas was suggested as improving the usefulness of the operating space and to permit continuous operation. It was said that the technical possibility of such a process was studied at a major laboratory in Germany, and became the subject matter of two German patents, German Pat. Nos. 666,712 and 690,714, but was discarded in favor of the vacuum method. I have found, however, that in a molten slag reaction system the use of a substantially static inert gas does not stop the reaction and results in significant improvements over the vacuum method.
A principal objective of the present invention is to decrease the concentration of silicon in the product magnesium. Magnesium as presently produced by the commerical Magnetherm process contains silicon as an impurity in a concentration of about 500-600 ppm--a level which is considered intolerably high by some experts, particularly for its use in the production of titanium, zirconium or the like. Typically, also, the Magnetherm product magnesium contains about 600 ppm manganese, 130 ppm iron, and about 250-600 ppm of remaining minor constituents, such as zinc, copper, tin, lead and nickel. It is a further objective of this invention to decrease substantially the concentration of these metallic impurities in the magnesium product.
One of the major drawbacks of the Magnetherm process, as mentioned above, is its requirement of operation under high vacuum. Many efforts have been made to adapt the process so that it may be operable at atmospheric pressure; see, e.g., my earlier applications referred to above and the Magee et al. patent, U.S. Pat. No. 3,441,402. These efforts involve in most cases variations of the magnesium ore, reducing alloy or slag composition in order to favor the principal reaction to promote magnesium formation at a higher partial pressure. Some of these changes, however, entail attendant disadvantages. For example, the reactivity of the ferrosilicon reducing alloy can be increased by the use of high purity silicon, as disclosed in my copending application filed concurrently herewith; but the increased silicon content of the alloy will, other things being equal, result in increased silicon in the magnesium product. It is therefore a further objective of this invention to permit using a high-silicon reducing alloy while at the same time decreasing the silicon content in the magnesium product.
The ferrosilicon reducing agent used in the Magnetherm process is fed to the reaction zone through a discharge pipe from which it is charged onto a highly agitated molten slag bath where the reaction takes place. Since this alloy is ground to a fairly small size, it carries with it a metallic dust. The oxidic materials fed to the reaction zone, e.g., dolomite, magnesia, bauxite or alumina, also contain dust. The conditions in the reaction zone are such that the agitation of the slag combined with the high velocity of the escaping magnesium vapor inevitably results in the carrying over of at least a small portion of this dust into the condenser, with a consequent contamination of the magnesium product.
The seriousness of this dusting problem becomes clearer upon the consideration that, under the high vacuum operation (about 7 to 35 mm. Hg) coupled with a high temperature (about 1500.degree. C), the magnesium vapor passing through the duct connecting the furnace and the condenser may reach a velocity on the order of 100 ft. per second. Consequently, it is a further objective of this invention to decrease the tendency for metallic and oxidic dust to be carried over from the furnace to the condenser.
U.S. Pat. No. 3,017,263, issued to Bretschneider et al., describes in some detail the contamination of magnesium both from vaporized metals such as silicon and from dusting, and a proposed method to combat it, by way of passing magnesium vapor through loosely heaped lumpy material kept in movement. It is a further objective of the present invention to purify the magnesium product without the use of moving solids.